i've got much the same idea, also partly mapped out in my mind how it is constructed so that it can survive a deluge by being mostly above the opening, and well sealed and dug into rock like the diving chamber of a submarine
and yeah, how to store the data... mapping out a whole scheme of recovering technology, stuff like "ok, here you make charcoal, then you get this red stone and build a furnace like this and ... next you have iron and copper, i feel like metallurgy is definitely a key technology to recover


